Inertial Measurement Unit (IMU)

Inertial Measurement Unit (IMU) is used in devices to measure velocity, orientation and gravitational force. Earlier, IMU consists of two sensors namely accelerometers and gyroscopes. The accelerometer is used to measure the inertial acceleration whereas gyroscope measures angular rotation. These sensors have three degrees of freedom to measure from three axes.

MEMS IMU
MEMS IMU

Types of IMU

IMU with Two Sensor — This type of IMU consists of accelerometer and gyroscope. Each sensor has two or three degrees of freedom (DOF) defined for x, y and z-axis, which on combining for both sensors adds up to four or six degrees of freedom. The value of acceleration measured from the accelerometer and angular velocity measured from gyroscope are noted separately, while the angle measured using both sensors are calibrated to get accurate data. IMU of this kind is not interfered by external magnetic field if present around the sensor. Its accuracy is affected due to sensors noise and drift issue of the gyroscope. 

Working Principle

IMU measures two to six DOF (Degrees of freedom represent a direction in which independent motion can occur). The maximum DOF it can measure is 6 which include 3 degrees of translation motion over 3 perpendicular axes and 3 degrees of rotational movement about 3 perpendicular axes (roll, pitch, and yaw). This six independent data together define the movement of an object or vehicle.

  • Pitch: also called the lateral or transverse axis, is the rotation of an object or vehicle fixed between side to side (right-left) axes.
  • Yaw: is the rotation around the vertical axis, lies perpendicular to an object.

Accelerometer

A mechanical accelerometer
A mechanical accelerometer
A mechanical accelerometer
Piezo-electric accelerometer
Piezo-electric accelerometer
Piezo-electric accelerometer

Gyroscope

Similarly, gyros are of different types, but in IMU MEMS-Based Gyroscopes (MEMS) are implemented. MEMS (Micro-Electro-Mechanical Systems) is a technology defined as miniaturized mechanical and electromechanical devices and structure made up of microfabrication techniques, its dimension varies from below one micron to several millimetres.

A vibrating mass gyroscope
A vibrating mass gyroscope
A vibrating mass gyroscope

Magnetometer

About 90% of the magnetometer works on the Hall Effect.

Diagram representing Hall Effect
Diagram representing Hall Effect
Diagram representing Hall Effect

Bit of Calibration

IMUs combine input data from several different sensors to accurately measure the movement. To get the accurate values calibration of the sensor is essential instead of using the raw data. Calibration parameters can be stored in the memory of an IMU and are automatically reflected in the resulted data. Calibrations are also accomplished by using a magnetometer which reduces orientation drift (errors which accumulate over time). Some device uses a proprietary sensor fusion algorithm to combine magnetometer and gyroscope data, determining the orientation of the device relative to a global frame of reference. Sensor fusion refers to processes in which signals from two or more types of sensor are used to update or maintain the state (orientation, velocity and displacement) of a system. A sensor fusion algorithm maintains this state using IMU accelerometer and gyroscope signals together with signals from additional sensors (like magnetometer) or sensor systems. The most popular techniques for performing sensor fusion is Kalman filters.

Applications

IMUs are used in motion sensing, unmanned navigation systems, vibration control, surveying, and various tracking systems. In unmanned navigation system (drone, aircraft), IMU is implemented to calculate altitude and relative position to a reference frame. In the manufacturing industry, IMU has been made to improve productivity. In the medical area, many IMUs has been currently used to help and assist human in surgical, or medical analysis. In robotics, some of the robots require linear and angular data for their movements; therefore the IMU is used to obtain the desired data.

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